Bacterial surface plays an essential role in regulating the interaction of microorganisms with the environment. Data from our laboratory and others suggest that lactoferrin (LF) may elicit antibacterial effects via modulation of certain surface structures. We have described antimicrobial effects of LF that are distinct from the traditionally considered iron deprivation properties of this molecule. In particular, our studies have identified specific LF-binding targets on the surface of many microorganisms. The outcome of this interaction can be varied depending on the test strain and can result in metabolic inhibition, bacteriostasis, and increased susceptibility to other antimicrobials and/or reduced interaction with surface (inhibition of colonization). We have observations that defined additional insights into the nature of bacterial surface targets for LF in various enteric bacteria. LF demonstrated specific binding to pore- forming proteins (porins) in the bacterial outer membrane and the magnitude of this interaction was directly related to the susceptibility of enterics to antibacterial effects of LF. Although enterics express porins, certain strains are resistant to LF effects and do not demonstrate specific LF binding. This LF "resistance" was attributed to the shielding of porin accessibility by the carbohydrate O-side chains of lipopolysaccharide (LPS). Fresh clinical isolates of virulent serotypes were more resistant to LF antibacterial effects than were their virulent counterparts. Similarly, mutants with progressive deletions in O-side chain and core polysaccharide demonstrated increased LF binding and progressive susceptibility to LF effects. LF has also been shown to have several effects that may influence the colonization of Escherichia coli. LF inhibits the expression of various bacterial fimbria and block bacterial interaction with host subepithelial matrix components such as fibronectin, fibrinogen, various collagens and laminin. Prophylactic oral administration of LF reduces the colonization of E. coli in the mouse intestine. The molecular mechanisms of these multiple effects are unknown. The overall objective of this proposal is to define the nature of milk LF interactions with Escherichia coli as well as other enterics and their response to this interaction.